Circuit devices often communicate data and control signals via serial buses, which may be bi-directional. In some instances, isolation circuitry may be provided to electrically isolate the circuit devices. One example of a circuit device that uses isolated serial buses to communicate data is a subscriber line interface circuit (SLIC), which can be coupled to communication lines, such as tip and ring lines of a telephone network, to send and receive information via the network communication lines. The SLIC may be adapted to communicate with another circuit via multiple pulse code modulation (PCM) buses and multiple serial peripheral interface (SPI) buses. Each bus typically includes an isolation circuit to electrically isolate the SLIC from the other circuit. Such isolation circuits can include capacitors, transformers, optical isolation circuits, other isolation circuits, or any combination thereof. However, such isolation circuits can increase the overall cost of a device. Hence, there is a need for an improved communication interface between circuit devices.